A gas turbine includes a rotor, on which various stages with compressor blades and turbine blades are provided, and of a stator housing. The rotor is mounted in bearings at each end of the rotor shaft.
The control of the gas flow inside the gas turbine is of paramount importance with regard to both the functionality and the effectiveness. Sealing techniques are used at various locations along the rotor shaft in order to reduce the axial gas flow along the shaft. This is especially important next to the bearings in order to prevent the oil which is used in the bearings from overheating due to the hot gases of the gas flow.
Two types of sealing techniques are traditionally used in this situation—usually alternatively, sometimes also in combination. These are labyrinth seals and brush seals.
Labyrinth seals have no metal-to-metal contact between the rotor and the stator; the sealing effect is therefore relatively small. However, they offer the advantage of low rotational friction and of a therefore virtually unlimited service life.
On the other hand, brush seals have higher friction losses on account of the friction between the bristle ends and the rotor shaft. This results in wear, which limits the service life of the seal. However, brush seals stem the axial gas flow more effectively, in particular in the case of higher axial pressure differences.
The use of these technologies for sealing in gas turbines has numerous restrictions. Firstly, the axial pressure difference that they can withstand is still fairly low. In the case of the brush seals, this is due to the bristles, which have the same stiffness in the axial and circumferential directions: high pressures can cause the bristles to blow back on themselves in the axial direction. The capability of the seals to allow a significant radial movement and to resist it is also low.
The design of a brush seal is often a compromise between the use of a supporting plate, which is intended to give sufficient axial support, and the non-restriction of the radial movement.
In order to avoid the disadvantages of the known brush seal, a leaf seal has been proposed in U.S. Pat. No. 6,267,381 B1, this leaf seal performing the same function as either a labyrinth seal or a brush seal but having the advantages of both. Instead of the bristles, which are produced from wires of circular cross section, thin metal leaves are assembled in a certain arrangement (see, for example, FIG. 2 of U.S. Pat. No. 6,267,381 B1 or FIG. 1 of the present application). The leaves, which are oriented with their surfaces essentially parallel to the axial direction, are much stiffer in the axial direction than in the circumferential direction. Thus the seal can withstand higher pressure differences without restricting their possibilities for allowing radial movements. The wide region on the rotor, which is swept by the tips of the leaves, provides the opportunity of producing a hydrodynamic force during operation, and this hydrodynamic force can separate the leaf tips from the shaft. In this way, a distance of a few microns can be produced and maintained, so that the wear, the friction heat and the friction losses can be reduced virtually to zero.
The basic design relates to a number of thin metal leaves which have a controlled air gap between them and are fastened at a predetermined angle to the radius. The air gap is a critical design parameter: it enables an air flow to occur in order thus to produce the hydrodynamic effect; however, it must not be so large as to allow an excessive axial leakage flow.
Two variants of leaf seal designs are possible; in the one variant the leaves are blown downward, but in the other they are blown upward. In the variant having the leaves blown downward, there is a distance between the leaf tips and the shaft during the assembly and start-up, and this gap is reduced to very small values by the use of an air flow between the leaves. On the other hand, in the variant having the upward blowing, there is slight mutual influencing between the leaf tips and the shaft during the start-up, and a distance is produced when the shaft is accelerated. In both cases, the flow of the medium through the air gaps between the leaves is critical, as is the control of the seal's inside diameter, which is produced by the leaf tips.
The air flow through the leaves can be varied by using a front and a rear plate which leave a narrow gap free between the surfaces of the leaf stack and the plates (see abovementioned FIGS. 1 and 3). A careful design of these geometries makes it possible to control the upward or downward blowing effects. It may also be desirable to assist the downward blowing effect by an active pressure feed along the length of the leaves or inward from the front side or from the rear directions.
One of the other main advantages of the leaf seal concept is a greater tolerance of the radial movement than in labyrinth or brush seals. This requires a large distance there between the inside diameter of the front and rear end plates and the shaft.
Depending on the geometry selected for the seal and on the diameter of the shaft to be sealed, the number of leaves can be several thousand or umpteen thousand. The accuracy with which said leaves can be produced, assembled and connected, in the course of which a reproducible air gap between each pair of leaves is ensured, is critical for the successful implementation of every possible seal design.
The joining method for fixing the leaves in their position could be a mechanical technique, such as clamping in place, welding or brazing or any possible combination thereof. It is quite obviously important that a minimum disturbance of the leaves or of their relative positions occurs during the joining process.